1. Field of the Invention
The present invention relates to a calibration source (light source) for calibrating a spectroradiometer that enables a user to easily perform spectral sensitivity calibration and wavelength calibration of the spectroradiometer without using a calibration source, e.g., a black body source, which requires provision and control of a strict calibration environment, as well as to a calibration method using the calibration source, and a calibration system.
2. Description of the Related Art
Heretofore, there have been widely known spectroradiometers for measuring and evaluating spectral radiances, radiances, and colors of various light sources and display devices. FIG. 14 is an illustration showing an arrangement of a spectral analyzer 70 in a conventional spectroradiometer. The spectral analyzer 70 is a so-called polychromator, in which light 701 to be measured is guided to an incident slit 712 through an objective optical system 711 for wavelength dispersion by a diffraction grating 713, and a wavelength-dispersed image of the incident slit 712 is formed on a photo detective sensor array 715 through an imaging optical system 714. Outputs from respective pixels of the sensor array 715 in accordance with a spectral intensity of the light 701 to be measured are sent to a control/processing unit 717 via a processing circuit 716 for conversion into spectral radiances. Thus, intensity distributions of all the wavelengths in a wavelength range to be measured are simultaneously measured.
There are two calibrations in the spectroradiometer, namely, wavelength calibration and spectral sensitivity calibration. The wavelength calibration is performed by storing a lookup table showing a correlation between the pixel number n of the respective pixels of the sensor array 715, and a center wavelength λn of the pixel having the pixel number n. The spectral sensitivity calibration is performed by storing a ratio of a spectral intensity obtained when an incandescent source such as a tungsten lamp whose spectral radiance is known is measured by a spectroradiometer to be calibrated, to the known spectral radiance, as a sensitivity correction factor.
The spectroradiometer has suffered from wavelength fluctuation due to a change of an optical arrangement of a spectral analyzer incorporated in the spectroradiometer, or spectral sensitivity fluctuation resulting from change of a diffraction grating, an optical component, or a circuit element through aging despite the fact that the wavelength calibration and the spectral sensitivity calibration are carried out by the manufacturer at the time of production or at the time of shipment. In view of this, it is necessary to re-calibrate the spectroradiometer with sufficient frequency to maintain the measurement precision of the spectroradiometer after the shipment.
The spectral radiant emittance M (λ, T) of an incandescent calibration source used generally and conventionally for spectral sensitivity calibration of the spectroradiometer is given by the following Plank's equation where C1 and C2 are constants:M(λ,T)=C1·λ−5exp [1−C2/(λ·T)]  (1)The spectral radiant emittance greatly depends on the color temperature T of the light source relatively and absolutely. In view of this, it is necessary to
(a) finely control the color temperature of the light source, or
(b) monitor the color temperature of the light source
to use the incandescent calibration source as calibration reference. In the case of (b), at least two reference sensors having different spectral sensitivities from each other are necessary to monitor the color temperature of the light source.
Assuming a case to maintain a radiant intensity at a wavelength of 400 nm of an incandescent calibration source having a color temperature of 2,800K within a fluctuation of ±1%, in the case of (a), it is necessary to control the temperature of the light source with a precision of ±2K, and in the case of (b), it is necessary to keep a wavelength fluctuation of a spectral sensitivity of the reference sensor provided with a filter having a temperature dependence or a time-based variation within 0.5 nm or less. Since the conventional calibration source requires special arrangement in maintaining precision, it is substantially impossible for a user to re-calibrate or correct spectral sensitivity of the spectroradiometer. In view of the above, it is a general practice for the user to return the spectroradiometer to a manufacturing factory, a servicing facility, or a like site for re-calibration of the spectral sensitivity thereof. However, a cost and time are required for the returning, and therefore, it is difficult to perform re-calibration with sufficient frequency.